CVID patients with gastrointestinal symptoms 10 years of age and older may be eligible for this study; CVID patients without gastrointestinal symptoms 18 years of age and older will be enrolled as control subjects. Candidates will be screened with a review of their medical records, a medical history and physical examination, HIV blood test, stool sample, and hydrogen breath test. The breath test measures the amount of hydrogen in the breath after drinking sugar water, showing the digestive effects of bacteria in the upper intestine.